PROJECT SUMMARY/ABSTRACT. Post-partum weight retention contributes to obesity for many women, increasing risk for cardiovascular disease and other chronic diseases and complicating future pregnancies. Lifestyle interventions have shown to be modestly efficacious for post-partum weight loss in randomized controlled trials, yet interventions with numerous visits are logistically challenging for many post-partum women. Innovative and efficacious treatment models for post-partum weight loss that fit into the busy lives of new moms are needed, and cost-effectiveness is critical for adoption. Facebook may be an efficient platform for delivering evidence-based weight loss programming to post-partum women. Delivering interventions via Facebook allows us to connect with post- partum women where they are, more fully integrating into their lives and daily routines. We have developed a post-partum weight loss intervention based on the Diabetes Prevention Program, tailored to needs of post- partum women and for delivery via Facebook. The goal of the proposed project is to gather critical preliminary data to finalize the design of a large randomized trial to compare the non-inferiority and cost-effectiveness of this post-partum weight loss intervention delivered via Facebook versus in-person group sessions. We will conduct a pilot randomized trial with 72 overweight or obese post-partum women comparing delivery of a post- partum weight loss intervention via Facebook to in-person group sessions. We will examine the feasibility of recruitment (especially the proportion of women unwilling to be randomized to one condition), sustained participation, contamination, retention, and feasibility of assessment procedures, particularly measurement of cost-related data, in both treatment conditions. We will describe weight loss as an exploratory outcome. Second, in pre-pilot and post-intervention focus groups, we will solicit women's feedback on posts with low engagement, and will iteratively refine these posts to make them more engaging. Finally, we will compare self- reported time spent on Facebook to participate in the intervention with application-tracked time on Facebook, changes in use, and time spent visibly engaging, to develop procedures for measuring time spent on Facebook to participate in the intervention that balance accuracy and participant burden. The proposed project will provide preliminary data needed to finalize the design of a subsequent large non-inferiority trial. Demonstrating cost-effectiveness in addition to efficacy of our Facebook-delivered post-partum weight loss intervention is critical to support widespread implementation.